1. Field of the Invention
The present invention relates to a process for forming a catalyst layer in an electrode for a polymer electrolyte fuel cell, and a process for producing a membrane/electrode assembly for a polymer electrolyte fuel cell.
2. Discussion of Background
Fuel cells are expected to be widely used in future, since they have high power generation efficiency, and the reaction product is only water in principle whereby the load to the environment is low. Among them, a polymer electrolyte fuel cell has a high output power density and is expected to be widely used for automobiles or as a distributed power generation system, a transportable power generation system or a household cogeneration system.
A polymer electrolyte fuel cell is usually constituted by a cell wherein an electrically conductive separator having gas flow paths formed, is disposed on each side of a membrane/electrode assembly comprising a cathode having a catalyst layer and a gas diffusion layer, an anode having a catalyst layer and a gas diffusion layer and a polymer electrolyte membrane disposed between the catalyst layer of the cathode and the catalyst layer of the anode.
In a case where the membrane/electrode assembly is poor in the dimensional stability or mechanical strength, the handling efficiency is likely to be poor at the time of assembling the cell, or the polymer electrolyte membrane is likely to break during the operation. Therefore, the membrane/electrode assembly is required to have sufficient mechanical strength and dimensional stability.
Further, recently, the polymer electrolyte fuel cell is required to be operated under a low humidity condition where the relative humidity of the reaction gas (fuel gas and oxidant gas) is low, in order to simplify the fuel cell system or to reduce the cost. If the power generation can be conducted stably under a low humidity condition, it will be unnecessary to provide a peripheral device such as a humidifying device, whereby it will be possible to reduce the size or cost of the fuel cell system. Accordingly, the polymer electrolyte membrane for the membrane/electrode assembly is required to have a high ion-exchange capacity (i.e. the equivalent weight (grams of the polymer per one equivalent of ionic groups, hereinafter referred to as EW) being small) and a thin thickness (at most 25 μm) in order to maintain the ion conductivity even under a low humidity condition.
However, the polymer electrolyte membrane has such a nature that as EW is small, it undergoes substantial swelling or shrinkage due to a change of the humidity environment. Such swelling or shrinkage occurs due to a change in the operation conditions such as the cell temperature, the relative humidity of the reaction gas, the amount of the reaction gas, the output power, etc. Accordingly, in a practical application, due to repetition of such swelling and shrinkage, the polymer electrolyte membrane is likely to undergo a dimensional change irregularly, and as a result, the polymer electrolyte membrane will get wrinkles. And, in a case where the thickness of the polymer electrolyte membrane is thin, the polymer electrolyte membrane may break due to such wrinkles.
As a polymer electrolyte membrane and a membrane/electrode assembly having the dimensional stability improved, the following ones have, for example, been proposed.
(1) A thin composite membrane (polymer electrolyte membrane) having a thickness of at most about 25 μm, having an ion-exchange resin impregnated to a stretch-expanded tetrafluoroethylene membrane having a fine porous structure (Patent Document 1).
(2) A composite membrane (polymer electrolyte membrane) having an ion-conductive polymer impregnated in a porous body of individual fibers which are randomly oriented (Patent Document 2).
(3) A membrane/electrode assembly having a reinforcing material containing electrically conductive nano fibers disposed on at least one side of a polymer electrolyte membrane (Patent Document 3).
However, the composite membrane (1) has a problem such that the ion-conductivity tends to be low as compared with a membrane not reinforced, and especially, the power generation performance is likely to be low under a low humidity condition.
Also the composite membrane (2) has a problem that when a porous body having sufficient chemical stability and mass-producibility is selected, the ion-conductivity tends to be low as compared with a membrane not reinforced, and especially, the power generation performance tends to be low under a low humidity condition.
With the membrane/electrode assembly (3), the dimensional stability and mechanical strength are still inadequate, and especially when the thickness of the polymer electrolyte membrane is at most 25 μm, it is not durable against the above-mentioned repetition of swelling and shrinkage. That is, in a case where a reinforcing material is to be provided on the outside of the polymer electrolyte membrane as in the membrane/electrode assembly (3), in order to increase the dimensional stability of the polymer electrolyte membrane, it is required to increase the bond strength between the reinforcing material and the layer adjacent thereto thereby to adequately reinforce the polymer electrolyte membrane by the reinforcing material.
As another problem of the membrane/electrode assembly, there is a problem that defects such as cracks are likely to form in the catalyst layer during the production of the membrane/electrode assembly. That is, the catalyst layer is formed by applying a coating fluid containing a catalyst and an ion-exchange resin on a substrate such as a release film, followed by drying, and cracks, etc. are likely to form in the catalyst layer due to shrinkage, etc. of the ion-exchange resin containing a solvent, or evaporation of the solvent during the drying. Further, the catalyst layer is brittle, and defects such as falling of the catalyst layer, transfer failure, etc. are likely to result due to cracking or the like of the catalyst layer at the time of the transfer to the polymer electrolyte membrane.
Patent Document 1: U.S. Pat. No. 5,547,551
Patent Document 2: JP-A-10-312815
Patent Document 3: JP-A-2006-252967